Waste water streams from industrial plants, including petroleum refineries, chemical plants, pulp and paper plants, mining operations and food processing plants, can contain toxic substances such as cyanides, sulfides, sulfites, thiosulfates, mercaptans, and disulfides that tend to increase the chemical oxygen demand (COD) of the waste water streams. Examples of these waste water streams in petroleum refineries include sourwater, sourwater stripper bottoms, and spent caustics. The Environmental Protection Agency (EPA) and various local agencies have placed limits on the allowable levels of these toxins in industrial waste water effluent streams.
The conventional methods for wastewater treatment include incineration, biological oxidation, and chemical oxidation using H.sub.2 O.sub.2, C1.sub.2, NaOC1, C1O.sub.2, and KMnO.sub.4. The concentration of the toxin in the waste water may be too low to treat economically using conventional means.
U.S. Pat. No. 5,112,494 to Yan teaches a method of removing cyanide from cyanide-containing waste water utilizing a water insoluble metal compound deposited onto a porous adosrbent substrate, the entire disclosure of which is incorporated by reference herein. The patent also teaches that a process using a water insoluble metal compound deposited onto a porous adsorbent substrate is effective in reducing the chemical oxygen demand of cyanide-containing waste water by oxidizing the oxidizable components of the water.
It is well known that sulfides in waste water, including sourwater stripper bottoms or foul water, can be oxidized using air to reduce the chemical oxygen demand of the waste water. These air oxidization processes are commonly practiced in petroleum refineries. In these air oxidation processes, the sulfides are oxidized to thiosulfate as is shown in the following representation: EQU 2S.sup..sup.50 +20.sub.2 +H.sub.2 O.fwdarw.S.sub.2 O.sub.3.sup..dbd. +20H.sup.-
S.sub.2 O.sub.3.sup..dbd. +20.sub.2 +H.sub.2 O.fwdarw.2SO.sub.4.sup..dbd. +H.sup.=
As noted in Abegg et. al. ("A Plant for Oxidation of Sulfide Containing Refinery Waste by Air", Ardol Kohle Erdgas Petrochemie, Sept. 1962), the reaction rate of sulfide to thiosulfate in the presence of air, as represented by equation (1) above, is relatively rapid. Unfortunately, the reaction rate of thiosulfate to sulfate as represented by equation (2) above, is extremely low, so that in an air oxidation process, most of the sulfides are converted to thiosulfate. A second, more severe process is required to oxidize the thiosulfate to sulfide. Based on Abegg's data, Beychok ("Aqueous Wastes from Petroleum and Petrochemical Plants," page 210 John Wiley, 1967) observed that, "To oxidize 34% of the sulfides to sulfates requires a tenfold increase in tower volume compared with units that oxidize the sulfides to thiosulfate." Thus, a catalyst is required to convert sulfides and thiosulfate efficitnely to sulfate.
Copper is an effective catalyst for oxidation of sulfides and thiosulfate. Beychok also observed that by use of homogeneous CuC1.sub.2 catalyst, sulfides can be converted completely to sulfates. Continuous addition of a homogeneous catalyst to the treatment system is undesirable because of the chemical and operating costs, and most importantly, pollution of the treated water by copper.
In developing water treatment processes, particular concern is directed to processes which do not leave residues in the treated stream. Residues can cause additional disposal problems. Materials consumption and cost is also an important factor; thus, it is important to avoid processes which require replenishing the supply of costly catalyst and reagent.